(1) Field of the Invention
The present invention relates to the art of poling an optical waveguide device for use in the field of optical communications, and more particularly to an optical waveguide device to be subjected to optical poling based on the application of both an ultraviolet radiation and an electric field, a method of manufacturing such an optical waveguide device, and a method of optically poling such an optical waveguide device.
(2) Description of the Prior Art
One type of optical waveguide device is available as a waveguide optical switch. The waveguide optical switch comprises an optical substrate and an optical waveguide made of a given material that is disposed on the optical waveguide. The waveguide optical switch performs a switching action by changing the intensity of light or changing light paths when the waveguide optical switch is caused to change its refractive index by a thermo-optic effect or an electro-optic effect. Waveguide optical switches whose optical waveguides are made of quartz have recently been expected to find practical applications because they suffer a small loss, allow quartz waveguides to be fabricated together on one substrate, and can be connected for good matching to a single-mode fiber of quartz.
A specific optical switch using a quartz waveguide that has come into reality is a TO (Thermal Optical) switch as introduced by N. Takao, et. al., xe2x80x9cSilica-Based Single-Mode Waveguides on Silicon and their Application to Guide-Wave Optical Interferometersxe2x80x9d, J. Light Technol., VOL. 6, 1988, pp. 1003-1010. However, the introduced TO switch has a response speed of about 1 msec, and is not suitable for high-speed signal processing applications.
One waveguide optical switch that can possibly be used as a high-speed switch is a waveguide optical switch whose response speed is increased by the Pockels effect that is induced by thermal poling to apply a high voltage at an increased temperature. The Pockels effect is described in detail by P. G. Kazansky, et. al. xe2x80x9cPockels effect in thermally poled silica optical fibersxe2x80x9d, Electronics Lett., Vol. 31, 1995, pp. 62-63.
The above article reports that the Pockels effect offers a response speed of 10 nsec or lower, allowing a high-speed switch operable at a frequency of 100 MHz or higher to be realized. However, the drive voltage for the high-speed switch is required to be 1 kV or higher because an electro-optical constant inducted by thermal poling has a small value of 0.05 pm/V or lower.
The Pockels effect can be enhanced by an optically pumped poling process which applies visible light or ultraviolet (UV) radiation while under an electric field. An article by T. Fujiwara, D. Wong, Y. Zhao, S. Fleming, S. Poole, and M. Sceats, Electron Lett., 31, 1995, 573 has reported that a high electro-optical constant of 6 pm/V is obtained by optically pumped poling.
Japanese laid-open patent publication No. 9-258151 discloses a waveguide optical switch based on optically pumped poling. FIG. 1 of the accompanying drawings schematically shows the disclosed waveguide optical switch.
The waveguide optical switch shown in FIG. 1 is a Mach-Zehnder interferometer waveguide optical switch which has two waveguides 112, 113, serving as Mach-Zehnder interferometer arms, disposed on Si substrate 111, with thin film electrode 116 disposed on one of waveguides 112. Waveguides 112, 113 have ends coupled respectively to two input waveguides as input ports P1, P2 by directional coupler 117, and other ends coupled respectively to two output waveguides as output ports P3, P4 by directional coupler 118.
The illustrated Mach-Zehnder interferometer waveguide optical switch is poled as follows: While a laser beam having a prespecified wavelength, i.e., such a wavelength that will not cause a coupling in directional couplers 117, 118, is being introduced from input port P1, a voltage of a certain magnitude is applied between thin-film electrode 116 and Si substrate 111. The laser beam introduced from input port P1 is not coupled in directional coupler 117, but propagated through waveguide 112 as one of the arms. After elapse of a predetermined time, the laser beam is turned off, and the voltage is dropped to 0 V, thus finishing the poling process.
The arm waveguides thus optically poled induces an electro-optic effect which allows the refractive index to change when an external electric field is applied. For example, the magnitude xcex94n of a change of the refractive index which is produced when an external electric field Eex is applied in a TM direction is expressed as follows:
xcex94nTE=(xc2xd)r1nTE2Eex 
xcex94nTM=(xc2xd)r2nTM3Eex 
(see Nishihara, et. al., xe2x80x9coptical integrated circuitxe2x80x9d published by Ohm-sha). In the above equations, r1, r2 represent electro-optic constants in the TE, TM directions, respectively, upon application of the external electric field in the TM direction, and nTE, nTM represent refractive indexes in the TE, TM directions, respectively. It will be seen from the above equations that the stronger the external electric field, the greater the change of the refractive index.
After the above poling process, a laser beam having a prespecified wavelength, i.e., such a wavelength that will cause a coupling in directional couplers 117, 118, is introduced from input port P1, and a voltage having a predetermined magnitude is applied between thin film electrode 116 and Si substrate 111. The laser beam introduced from input port P1 is coupled in directional coupler 117, propagated through waveguides 112, 113, then coupled in directional coupler 118, and propagated through the output waveguides of output ports P3, P4. FIG. 2 of the accompanying drawings show how the intensities of output beams from output ports P3, P4 change depending on the applied voltage. It can be seen from FIG. 2 that the phase of the output beams changes in substantial proportion to the applied voltage V.
As described above, it is possible to increase electro-optic constants and lower drive voltages according to the optically pumped poling process. However, the waveguide optical switch disclosed in the above publication which is processed by the optically pumped poling process suffers the following shortcomings if a UV radiation is used as the pumping radiation:
When the waveguide optical switch is UV-poled by introducing the UV radiation from input port P1 and applying a voltage of a predetermined magnitude between thin film electrode 116 and Si substrate 111, the introduced UV radiation is propagated through a Ge-doped waveguide to a region of the waveguide 112 which is to be pumped. Before the UV radiation reaches the region of the waveguide 112 which is to be pumped, the propagated UV radiation is partly absorbed by the Ge-doped waveguide. Since the UV radiation is progressively attenuated as it travels through the waveguide, the disclosed waveguide optical switch cannot efficiently be UV-poled, and fails to provide a uniform electro-optic effect. In addition, the waveguide which has absorbed the UV radiation tends to be damaged or otherwise made defective.
It is an object of the present invention to provide an optical waveguide device which has waveguides less susceptible to damage upon being irradiated with a UV radiation and which can efficiently be UV-poled, a method of manufacturing such an optical waveguide device, and a method of optically poling such an optical waveguide device.
To achieve the above object, there is provided in accordance with the present invention an optical waveguide device comprising a waveguide whose refractive index changes can be controlled by an electro-optic effect and a guide waveguide for coupling or applying an ultraviolet radiation to a predetermined area of the waveguide. The waveguide may comprise first and second waveguides serving as respective arms of Mach-Zehnder interferometer, and the guide waveguide may be arranged to couple or apply an ultraviolet radiation to a predetermined area of at least one of the first and second waveguides.
According to the present invention, there is also provided a method of manufacturing an optical waveguide device, comprising the steps of providing, on an optical substrate, a waveguide whose refractive index changes can be controlled by an electro-optic effect, and a guide waveguide for coupling or applying an ultraviolet radiation to a predetermined area of the waveguide, forming an electrode for generating an electric field having a predetermined magnitude, on a predetermined area of the waveguide, and applying an ultraviolet radiation to the predetermined area of the waveguide through the guide waveguide while a DC voltage having a predetermined magnitude is being applied to the electrode. The method may further comprise the step of removing the guide waveguide after the ultraviolet radiation is applied to the predetermined area of the waveguide. The step of forming the electrode may comprise the step of forming an electrode for controlling the refractive index changes of the waveguide due to the electro-optic effect.
According to the present invention, there is further provided a method of optically poling an optical waveguide device, comprising the steps of applying an electric field to a predetermined area of a waveguide disposed on a substrate and coupling or applying an ultraviolet radiation to the predetermined area of the waveguide through a guide waveguide made of a predetermined material. The waveguide may comprise first and second waveguides serving as respective arms of Mach-Zehnder interferometer, and the electric field may be applied to a predetermined area of at least one of the first and second waveguides and the ultraviolet radiation may be coupled or applied to the predetermined area of at least one of the first and second waveguides through the guide waveguide.
In the optical waveguide device and the method of optically poling the optical waveguide device, a directional coupler may be constructed of a portion of the waveguide and a portion of the guide waveguide, and the ultraviolet radiation propagated through the guide waveguide may be coupled to the predetermined area of the waveguide by the directional coupler.
The ultraviolet radiation radiated from an exit end of the guide waveguide may be applied to the predetermined area of the waveguide.
The guide waveguide may have an exit end facing a side of the waveguide, and the ultraviolet radiation propagated through the guide waveguide may be focused onto the predetermined area of the waveguide by the lens.
The guide waveguide may have a grating in a portion thereof, and the ultraviolet radiation propagated through the guide waveguide may be reflected to the predetermined area of the waveguide by the grating.
The grating may comprise a grating whose refractive index changes increase along the direction in which the ultraviolet radiation is propagated.
The guide waveguide may have a deflector in a portion thereof, and the ultraviolet radiation propagated through the guide waveguide may be deflected to the predetermined area of the waveguide by the deflector.
With the above arrangement, since the ultraviolet radiation can be coupled or applied to the predetermined area of the waveguide by the guide waveguide, any damage to the waveguide can be smaller than with the conventional optical waveguide device.
The guide waveguide is made of a material having high UV permeability such as SiO2, for example. Therefore, the ultraviolet radiation is not liable to be greatly attenuated by absorption when propagated through the guide waveguide.
If the ultraviolet radiation is coupled by the directional coupler and refractive index changes of the portion of the guide waveguide which has the grating increase along the direction in which the ultraviolet radiation is propagated, then the guide waveguide is capable of uniformly coupling or applying the ultraviolet radiation to the desired area of the waveguide, allowing the waveguide to obtain a uniform electro-optic effect.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.